The present invention relates to a two cycle internal combustion engine and, more particularly, a two cycle engine having an improved lubricating system for the cylinder and piston assembly.
Known lubricating systems for two cycle internal combustion engines are broadly divided into two types, which are a mixture lubricating system which supplies the lubricant in the form of a mixture with a fuel and an independent lubricating system in which the prescribed amount of the lubricant is supplied independent of the fuel with use of an oil tank and oil pump. In comparison to the latter system, the former is more advantageous in that it can dispense with a complex lubricant feeding device, and therefore it is more widely employed in internal combustion engines of a relatively small size in particular.
On the other hand, however, the mixture lubricating system has some difficulties: For example, with that system, it is difficult to have a sufficient amount of the lubricant be supplied effectively over an area where lubrication is required. Also, since the content ratio of the lubricant in the fuel-lubricant mixture is then constant irrespective of a variation in the load condition, the lubricant has to be contained in the mixture at a more or less higher ratio than required in preparation against a piston seizing, whereby it is likely that the lubricant cannot be completely burnt and that an unburnt portion of the lubricant can be discharged out of the engine together with the exhaust gas. Whereas it can take place also in the case of the independent lubricating system, such releasing of unburnt lubricant out of the engine, if occurs, gives rise to water pollution in the case of outboard engines or atmospheric pollution in the instance of engines for land vehicles.
In view of the above, it has of late been proposed to reduce the content ratio of the lubricant in the fuel-lubricant mixture so as to accordingly reduce the amount of unburnt lubricant discharged. However, to lower the content ratio of lubricant in the mixture inevitably accompanies an insufficient lubrication possibly to occur in various portions of the engine, particularly between the cyclinder and the piston, which then tend to undergo a piston scuff. Piston scuff occurs particularly about such portion of the cylinder as located at the sides of an exhaust port and away the same port toward the bottom of the cylinder.
Conventionally, it has been proposed as means for cancelling the above indicated difficulties to let the portion of lubricant accumulated in the crankcase be recycled to the inside of the cylinder through a lubricant outlet port provided in the inner wall surface of the cylinder by utilization of the pressure variation caused within the crankcase responsive to the engine operation, and thereby effect lubrication for the piston. However, technically it involves high difficulties to process to provide by drilling a lubricant outlet port at a necessary portion of the cylinder inner wall surface. Particularly, it is extremely difficult to have the outlet port drilled in the vicinity of the exhaust port or, more specifically, at a lower portion adjacent the exhaust port where piston scuff is highly likely to take place. Thus, conventionally the processing for the formation of a lubricant outlet port can be worked only at a limited portion or portions of the cylinder, and it accordingly is difficult to effect a sufficient and uniform lubrication for the piston and cylinder assembly.